The action potential after-hyperpolarization was investigated in the sympathetic ganglion of the bullfrog. Action potentials were elicited in three ways: by antidromic stimulation across the sucrose gap and by orthodromic stimulation through two separate presynaptic pathways. The effects of calcium ions and theophylline were studied on the electrogenic processes involved in action potential generation. We found that the spike after hyperpolarization in these neurons has a calcium-sensitive component. The calcium-sensitive component was found to be an increase in potassium permeability. This calcium-sensitive potassium conductance was potentiated by theophylline. Although, theophylline inhibits phosphodiesterase, the potentiation of the Ca activated K permeability was found to be independent of cyclic nucleotide elevation. The results indicate that the neuronal action potential has a Ca activated K permeability component that is modulated by theophylline independently of phosphodiesterase inhibition.